Semiconducting materials are used in a variety of applications, and may be incorporated, for example, into electronic devices such as photovoltaic devices. Photovoltaic devices convert light radiation into electrical energy through the photovoltaic effect.
The properties of semiconducting materials may depend on a variety of factors, including crystal structure, the concentration and type of intrinsic defects, and the presence and distribution of dopants and other impurities. Within a semiconducting material, the grain size and grain size distribution, for example, can impact the performance of resulting devices. By way of example, the electrical conductivity and thus the overall efficiency of a semiconductor-based device such as a photovoltaic cell will generally improve with larger and more uniform grains.
For silicon-based devices, silicon may be formed using a variety of techniques. Examples include silicon formed as an ingot, sheet or ribbon. The silicon may be supported or unsupported by an underlying substrate. However, such conventional methods for making supported and unsupported articles of silicon have a number of shortcomings.
Methods of making unsupported thin semiconducting material sheets, including silicon sheets, may be slow or wasteful of the semiconducting material feedstock. Unsupported single crystalline semiconducting materials can be produced, for example, using the Czochralski process. However, such bulk methods may disadvantageously result in significant kerf loss when the material is cut into thin sheets or wafers. Additional methods by which unsupported polycrystalline semiconducting materials can be produced include electromagnetic casting and ribbon growth techniques. However, these techniques tend to be slow and expensive. Polycrystalline silicon ribbon produced using silicon ribbon growth technologies is typically formed at a rate of only about 1-2 cm/min.
Supported semiconducting material sheets may be produced less expensively, but the semiconducting material sheet may be limited by the substrate on which it is formed, and the substrate may have to meet various process and application requirements, which may be conflicting.
Useful methods for producing unsupported polycrystalline semiconducting materials are disclosed in commonly-owned U.S. Provisional Patent Application No. 61/067,679, filed Feb. 29, 2008, titled “Method of Making an Unsupported Article of a Pure or Doped Semiconducting Element or Alloy,” and U.S. Patent Application No. PCT/US09/01268, filed Feb. 27, 2009, titled “Methods of Making an Unsupported Article of Pure of Doped Semiconducting Element or Alloy,” the disclosures of which are hereby incorporated by reference.
As described herein, the inventors have now discovered additional methods by which supported and unsupported articles of semiconducting materials may be made. The disclosed methods may facilitate formation of exocast semiconducting materials having desirable attributes such as improved crystal grain structure, reduced concentrations of impurities and/or defects, low surface roughness, and uniform thickness while reducing material waste and increasing the rate of production.